Your boiler's small pilot light is on, but the main burner is failing to ignite, meaning your heating and hot water will not work.

Main burner fails to light, pilot burning - S.I.T. control solenoid valve failed.

What causes this fault?

This fault occurs when the gas valve receives a signal to open the main burner but fails to do so physically. The most common reason is a mechanical or electrical failure of the solenoid coil within the S.I.T. gas control valve, which prevents gas from reaching the main burner even though the pilot light is lit.